In a telephone system it is required that the on-hook/off-hook status of subscriber telephone sets associated with the system be constantly monitored so that the system can properly respond, i.e., return dial tone, end ringing, or receive dial pulses, as quickly as possible after the subscriber lifts the telephone handset to initiate a call, or answer ringing or after the subscriber terminates a call by replacing the handset. In order to be able to determine the status of subscriber lines, a detection circuit is associated with each line and in the past many different types of circuits employing different devices, including photon or optical coupling, have been used. Typically, in an optical detection circuit, a photon-emitting device is connected in each of the tip and ring leads associated with the subscriber's telephone set. The photon-emitting devices, usually light-emitting diodes, are each light coupled to a photo-receiving device which are directly connected in a line scanning circuit forming part of the common control equipment of the telephone switching system. When the subscriber goes off-hook, the light-emitting diodes in the tip and ring leads become conductive, emit light which triggers the light coupled receiving devices in the scanning circuit thereby signaling the control equipment that the subscriber has gone off-hook. Similarly, the light coupled devices indicate when the subscriber has gone on-hook.
Although this type of light coupled circuit operates satisfactorily in most telephone switching systems, it does present problems in systems such as Private Branch Exchange (PBX) and key telephone systems where the detection circuit and common control equipment are located on the subscriber's premises, and the permissible system size becomes a critical issue. Since the prior art detection circuit associated with each subscriber line requires at least four light coupled devices, one light-emitting device in each of the tip and ring leads, and corresponding light-receiving devices, any reduction in the number of devices required for each line would result in savings of equipment mounting space and material cost.
Furthermore, optically connected devices which are connected in the tip and ring leads of a subscriber's line often present electrical balancing and transmission problems which must be overcome in order to provide satisfactory telephone service over the line.